Question

how did the world came to know about the holocaust happen in Germany

Answer 1

media and people ....

Answer 2

Main sources included the
Journals and diaries of the suffering Jewish wrote
Some of the Jews escaped the concentration camps and lived to tell the stories
When Hitler died, the records of the mass murders and accounts were aquired

One of the important sources was Anne Frank's diary. It gave some major information of Holocaust from a Jews point of view